One buff per stat, yes. That's also what the OP claims. You're not arguing that it's intended that wearing a belt makes armor useless, are you?
I don't think it's meant that way. The belt should only override the breastplates DT in terms of fire and ice damage, not in terms of physical damage, as the OP claims.
The more important question is whether this is a just an error in displaying of the DT in the menu or if it also affects combat the way it is displayed.
Yeah, this is what I meant. If I'm wearing armor that gives me +2 DT and a belt that gives +5 DT against fire, I should have +5 DT (fire) / +2 DT (everything else). Instead I get +5 DT (fire) / +0 DT (everything else), since the belt effect suppresses the armor effect completely instead of just overriding it when applicable.
And yes, it seems to affect combat: it shows up in the combat log's damage breakdown and after running the same encounter a few times (my PC solo vs. 3 stone beetles, auto attack only, no player input) I seem to be pretty consistently taking more damage with the belt than without it (~50% of my character's stamina with the belt and ~40% without).